Wind speed measurements on wind turbines are known to take place with a cup-anemometer or with a sonic-anemometer, which is normally mounted on a rear-part of a wind-turbine-nacelle. Because of this mounting position the wind-speed is measured, when the wind has already passed the rotating blades of the wind-turbine.
This wind-speed can not be used to change settings of the rotating blades, in order to optimize and thus reduce the wind-load acting on the rotating blades or optimize the output-power of the wind-turbine.
Modern wind-turbines have rotor diameters, which is defined by the rotating blades, of more than 100 meter. The changes of the wind speed over the rotor of such turbines may be considerable. It is therefore even more important for this kind of wind-turbine, to know the speed of the wind over the whole rotor before it passes the rotating blades in order to successfully optimize the turbine's operation.
So called “five-hole pitot-tubes” are used for wind-speed-measurements. The pitot-tubes are located at a leading-edge of a blade and the tubes are placed at a distance of one to two chords from the leading edge. The pitot-tube typically has a semi-spherical head, which is equipped with five surface-holes. These holes are arranged on two perpendicular rows and are connected with small tubes. At the end of the tubes there are pressure-transducers mounted.
This method is complex and expensive, providing more or less precise wind-speed-information. Especially the pitot-tubes are exposed to salt and dirt and in general contamination from the atmosphere, so their wind-speed-results will in general be more influenced than when using them during wind tunnel applications and may be worse than expected.